Condition indicating systems incorporating passive infrared sensors are used for space protection (workplace safety), as antitheft devices, such as in museums, and as alarm devices mounted on large trucks for sounding an alarm signal if a person approaches the vehicle. An alarm system incorporating an infrared sensor arrangement is described, for example, in German patent application P 38 37 054.9. In these systems, passive infrared sensors perceive heat sources in a particular monitoring area of the infrared sensor arrangement, and the output signal of the sensors is processed to produce a triggering signal which is applied to an alarm or other condition indicating control circuit.
To promote operational safety and to prevent tampering, it would be desirable to provide continuous fail-safe testing during operation of an infrared sensor to ensure that the unit is operative. For example, if the sensor of an infrared sensor fails, or if the window is blocked, the sensor arrangement and condition indicator will be essentially blind. If this should happen, occurring conditions are no longer detected and indicated by the system.
Failure of active-type sensors such as sonic transmitters is inherently detected during operation because transmitted signals are not received. However, it has not been possible in passive infrared sensor arrangements to provide a truly fail-safe, tamperproof system.
German patent application P 39 09 073.6 shows a process for continuous fail-safe testing of an infrared sensor arrangement of a condition indicator, in which the basic signals of the infrared sensor attributed to noise are greatly amplified and converted to digital signals, whose time occurrence is monitored. If a failure of the digital noise signals over a predetermined time interval is established, a failure indication is triggered. In this process test stimulation signals can also be triggered on the infrared sensor at regular intervals.